The present invention relates to composite articles, and more particularly to core densification.
Some composite articles incorporate a core such as honeycomb (HC) or foam for fabrication of various aerospace structures, such as panels, due to these core's advantageous strength to weight ratio. Such core composite articles include upper and lower composite skins, i.e., fiber reinforced resin matrix laminates that are separated and stabilized by the core layer.
In areas where fasteners are to be located, the core must be densified by either filling the HC core with an epoxy syntactic material or the incorporation of densely packed vertical pin densification for a foam type core. Either core densification limits fastener size to avoid core crush.
The HC core densification may require expensive hand work and is relatively heavy in weight. Provisions for fastener installation in composite sandwich structures are typically accomplished with an epoxy syntactic material at 45 lbs. per cu. ft. (pcf) to fill cells in a typical 3.0 pcf HC core. HC core details are often locally densified in a separate operation that may require machining and special bond preparation before the core detail can be assembled into a sandwich laminate. Furthermore, densification of curved core details may require significant tooling to maintain curvature during densification.
Vertical pin densification in foam type cores such as X-Cor™ or K-Cor™, is a pin insertion process typically performed at the core manufacturer. Pin densification in a 3.5 pcf or higher core is limited to 17 pcf as vertical pins may begin to interfere with X-Cor™ or K-Cor™ pins. A 17 pcf vertical pin densification with 4 ply face skins limits fastener size to approximately 5/32 inch diameter. Clamp-up loads for larger diameter fasteners may still crush the densified core and generally should not be utilized with vertical pin densification. The separate vertical pin insertion operation when manufacturing the core increases machine time. Furthermore, at highly loaded fastener locations in flare attachment areas, vertical pin densification may not efficiently transfer shear loads into the core.
Accordingly, it is desirable to provide localized densification of lightweight composite articles, e.g., to react significantly through fastener load.